Various means have been used in the prior art for determining the dynamic pressure that a blast wave produces on a target. The loading on a target produced by a blast wave usually may be described in terms of two phases. First a diffraction phase occurs which involves a phenomena produced when a shock front encounters and engulfs a target. A drag phase occurs after the rapid pressure variations associated with the diffraction process have ceased and a quasi-steady flow has been established over the target. The loading due to one or both of these phases may produce damage to a target, and therefore must be defined for calculating the effect upon a target or for correlating an effect with previous test results.
In order to determine the drag phase loading on a target, the dynamic pressure is required as a function of time at the target location. The dynamic pressure is generated by the air flow occuring in the blast wave. Dynamic pressure may be derived from separate measurements of stagnation overpressure and side-on overpressure versus time. The problem with prior art devices has been that they have introduced large errors in the determination of dynamic presure, particularly at shock front overpressures less than 10 psi. The prior art technique involved the use of independent measurements of stagnation overpressure and side-on overpressure from gauges which are physically separated to produce pressure measurements. At low side-on pressures the errors in the measured signals may become as large or larger than the magnitude of the dynamic pressure of interest. Thus the value of the differential pressure (Dq) and hence dynamic pressure (Pq) calculated from independently measured value of stagnation overpressure (Pt) and side-on overpressure (Ps) may be very much in error, with the error ranging from 30 to 100 percent in regions of particular interest.
The dynamic pressure (Pq) may be expressed in terms of the differential pressure (Dq) and side-on overpressure (Ps) by the following equation: ##EQU1## where .gamma.=ratio of specific heats
Ps=side-on overpressure PA1 Pa=ambient atmospheric pressure PA1 Dq=differential pressure PA1 Pt=stagnation overpressure
and where EQU Dq=Pt-Ps (2)
The present invention permits the use of a single sensor to measure the differential pressure (Dq) versus time generated by the air flow in a blast wave, another gauge to measure side-on overpressure Ps versus time, and then to calculate dynamic pressure (Pq) versus time from these measured valves using Equation (1). Because the differential pressure (Dq) is nearly equal to dynamic pressure (Pq) at low pressures, the change from the value of the differential pressure (Dq) produced by the use of Equation (1) is small, and is relatively insensitive to the value of side-on overpressure (Ps) used.
The problem with differential pressure gauges now available is that they are not designed to measure differential pressure in a blast wave. Some prior art gauges have a cylindrical cavity divided into two parts by a diaphram with inlet tube to provide a path for fluid to the cavities on either side of the diaphram. However, these gauges are unsuitable for measurement in the blast wave application because of the slow frequency response produced by the necessity of fluid flowing through the inlet tubes and filling or leaving the cavities.